The invention relates to a power supply for cathodes of electron beam guns disposed in vacuum chambers, especially those of vapor depositing guns, and provided with magnetic beam deflection or focussing, consisting of a transformer having a primary winding and a secondary winding, leads introduced into the vacuum chamber, and phase shift controlled electrical operating circuits for control of the heating current.
In electron beam guns whose cathodes must fundamentally be disposed in a vacuum, it is important on the one hand to keep the beam current, the accelerating voltage and the magnetic force acting on the beam for the purpose of deflection and/or focussing as constant as possible, but on the other hand it is also important to adapt these magnitudes very rapidly to changing operating conditions. For example, the accelerating voltage and the magnet field strength are in an inverse relationship to one another with regard to the path of the beam and hence with regard to the point of impingement of the beam. Any change in the magnetic field which results in a field change or even a field distortion will change the position of the point of impingement. For reasons of the life expectancy of the cathode, among other things, the cathode is supplied with a low heater voltage of a few volts at a high current. The heating power determines the temperature and hence the emission current of the cathode at a given acceleration voltage. Added to the high heater current is the necessity of keeping the cathode at the high accelerating voltage potential. This leads to special insulation problems.
In the case of vapor depositing guns, furthermore, the rate of vaporization of the material being vaporized depends to a great extent on a constant beam management. Thus, for example, the rate of vaporization can be reduced by as much as 20% by instabilities in the deflection system, which can be produced, for example, by magnetic field superimpositions, on account of the high heating current at the main frequency without any change in the beam power itself. This is probably to be attributed to a persistent change in the point of impingement of the beam. In the case of power supplies for vapor depositing guns, it can also happen that the insulating value of insulators can be impaired by the condensation of conductive vapors, e.g., in the case of the vaporization of metals, so that flashovers can occur. On account of the danger of flashover it is also impractical to install transformers in the vacuum.
The state of the art includes a power supply such as described in the beginning, in which the transformer is constructed as a main frequency transformer, is disposed outside of the vacuum chamber, and is connected to the cathode by coaxial cables through high-voltage and high-current lead-throughs. For a heating voltage of 7 volts and a heating current between 25 and 100 amperes, the secondary voltage of the transformer must be, for example, 14 volts, since about half of the heating power is lost due to ohmic resistance in the lines from the transformer to the cathode, and this despite large cross sections in the lines. The line losses alone necessitate a doubling of the parameters of the transformer design. In addition, the transformer has to be constructed with high-voltage insulation, since the secondary side, which is connected to a high voltage (acceleration voltage), has to be reliably insulated from the primary side which is at the main potential. The high voltage on the secondary side is also the reason for the use of the coaxial cable, whose outer conductor is at ground potential for safety reasons.
In the known system, furthermore, the primary side of the transformer is controlled through phase shift controlled thyristors. This requires that the transformer be greatly oversized to prevent it from operating in the saturation range, in order to protect the thyristors. If it is desired to avoid such oversizing, circuitry for the protection of the thyristors must be provided, the cost of which is equal to that of an inverter. The over-design factor to be preferred for the protection of the thyristors, combined with the overdesign to be provided to compensate for the line losses, results in transformers which are approximately four times the size that would be required theoretically for the cathode heating.
In the known power supply system, there also exist stray inductances as well as capacitances due to the coaxial cables, which endanger the electronic controlling circuits upon the occurrence of the virtually inevitable gun short circuits, in which spike pulses can be produced which can amount to a multiple of the working voltage.
An attempt has therefore been made to rectify the heating current on the secondary side and to provide a choke coil and a filter circuit in the secondary circuit. Conditions can be slightly improved in this manner. Nevertheless, the point of impingement or focal point of the electron beam is affected by the heater current and the emission current, respectively. To aid in the understanding of this, it shall be explained that the lines carrying the high heater current inevitably have to run, depending on the gun design, in the more or less close vicinity of the magnetic deflection field. The magnetic fields surrounding the conductors are added to the magnetic deflection field, necessarily resulting in a displacement of the focal point. Such superimposition of magnetic fields is utilized in known electron guns for the purpose of intentionally deflecting the electron beam, with the use of additional means. The undesired influence of such fields, however, must be compensated in the known power supply system by modifying the deflection. Furthermore, rectification on the secondary side entails an additional lengthening of the conductors, which in turn again increases the losses.
Disclosure has been made of an attempt to reduce the line losses and hence the size of the transformer by locating the transformer directly underneath the vacuum chamber. The shortening of the conductors which is achieved in this manner does reduce the line losses accordingly, but the rest of the problems remain unsolved. In particular, high-voltage lead-throughs cannot be eliminated, and the power cutoff spikes are reduced only slightly.